The capability of remote polling of a sensor is necessary in many kinds of application, especially wherever it is problematic to establish a permanent physical connection between a sensor and an associated evaluation unit, by way of which connection output signals of the sensor can be transmitted to the evaluation unit. Such connection problems arises wherever the sensor is moved relative to the associated evaluation unit, especially when rotary motions are involved. Examples of this that can be given are detecting the pressure in a pneumatic tire mounted rotatably on a vehicle, or measuring the torque on a rotating shaft.
These applications require the transmission of output signals of the sensor electromagnetically, in the broadest sense; that is, the transmission of radio signals, microwave signals, or light signals. One possibility of doing so is to equip the sensor element with its own power supply, to furnish the energy needed for the measurement and for transmitting the output signals. However, this principle quickly reaches its limits because of the attendant costs (battery), the relatively high weight of the sensor unit, and the requisite maintenance, since the battery has to be replaced after a certain time in operation.
It is therefore desirable to make the sensor entirely passive, or in other words to embody it without its own power supply, in order to circumvent the problems associated with the battery and to make the sensor smaller, lighter in weight, and less vulnerable.
One example of a sensor system with sensors that can be remotely polled electromagnetically is discussed in German Patent DE 19 702 768 C1. The sensor system known from this reference includes the following:                a sensor, disposed on the rotating object and sensitive to the dimensional variable, and means for forwarding the signals of the sensor to a processing device, which means include an antenna array for supplying the at least one sensor with high-frequency energy and for receiving a high-frequency signal, modulated as a function of the variable to be detected, from the sensor.        
This sensor system is suitable for detecting dimensional variables of the rotating object that are essentially constant throughout the entire object, so that the precise location where a measurement is made is not critical.
However, if it is critical to detect dimensional variables whose values are not uniform throughout the object, then the known sensor system rapidly reaches its limits. Measurements can still be performed in subregions of the rotating object if these subregions rotate jointly with the object, or in other words if the sensor can be disposed at the subregion of interest and can rotate jointly with it; but if it is critical to detect dimensional variables in a subregion of the rotating object that is stationary relative to a coordinate system that does not rotate with the object, then the known system is taxed beyond its capabilities.